In typically British fashion we use both quite readily.

Both are used, and it's fairly arbitrary which is used - bus/train times are usually 24-hours, general speech is usually 12-hour unless referring to something that is already in 24-hour time (but even then some people will switch them to 12-hour), and I see appointment cards, invoices etc. using both.

It's definitely in keeping with how we mix meters/feet and pints/litres.

From what I gather watching UK TV there's also the charming and efficient use of "half X" to refer to thirty minutes after hour X.